[InstCombine] Add log-pow simplification for FP exponent edge case. (#76641)

Fixes https://github.com/llvm/llvm-project/issues/76549

The cause of the optimization miss was -
1. `optimizePow` converting almost integer FP exponents to integer, and
turning `pow` to `powi`.
2. `optimizeLog` not accepting `Intrinsic::powi` as a target.

This patch converts constantInt back to constantFP where applicable and
adds a test.